Despite a vast literature showing that childhood sarcoma cells utilize insulin-like growth factors (IGFs) to maintain autocrine and paracrine-driven proliferation, antibodies that block ligand binding to the type IGF-1 receptor (IGF-1 R) have proven disappointing in the clinic. Our data show that one effect of IGF-1 R-targeted antibodies is to inhibit angiogenesis and sarcoma cell proliferation. However, tumor-secreted IGF-2, signaling through the insulin receptor (IN-R) circumvents these effects. We will take a candidate gene approach to test whether tumor sensitivity to IGF-1 R-targeted antibody therapy can be predicted from the expression of IGF/IN receptors and ligands, before treatment or after treatment. We will take a less-biased approach by screening a receptor tyrosine kinase siRNA library to identify receptors that may confer resistance to antibody treatment. In Aim 2, we will pursue approaches to enhance the antitumor activity of IGF-1 R-targeted antibodies by blocking IGF-2 signaling using ligand binding antibodies, small molecule inhibitors of IGF-1 R/IN-R, or Akt signaling. Preliminary results demonstrate that IGF-2 robustly activates STATS signaling via TOR in both vascular endothelial and sarcoma cells. Further, STATS cross-talks with N F - K B , consequently, we will evaluate combinations of IGF-directed antibodies combined with inhibitors of these pathways and determine their effects on angiogenesis and tumor cell proliferation in vitro and in sarcoma xenograft models. In Aim 3, we will explore the mechanism(s) by which IGFs protect against apoptosis induced by TOR inhibitors in some sarcoma cells. Our previous data showed that IGF-1 induces phosphorylation of BAD, through an Akt- independent pathway in vitro, and IGF-IR targeted antibody suppressed this in vitro and in a Ewing sarcoma xenograft model leading to rapamycin-induced apoptosis. We will explore how IGF-2 can protect cells from rapamycin or TOR kinase inhibitors, and determine whether IGF-2 protection is mediated through STATS signaling, and whether this can be inhibited by antibodies that block IGF-2/IN-R signaling or by inhibitors of STATS. pProject 3 is highly interactive with Project 2 (STATS signaling), impacts the role of N F - K B / S T A T S signaling in Project 1 and relies heavily on Cores (1-3). RELEVANCE (See instructions): Insulin-like growth factor signaling is dysregulated in each sarcoma histotype being studied. Work proposed will elucidate the mechanism(s) of intrinsic and acquired resistance to IGF-1 R-targeted antibody therapy, and test potential combinations that will overcome or reverse this resistance. Our studies will also identify pathways by which IGFs protect sarcoma cells from apoptosis, and examine strategies for selectively sensitizing vascular endothelial cells and sarcoma cells to undergo apoptosis in response to IGF-IR block.